Alkylation processes for the production of high octane gasoline cuts have been driven by the increasing demand for high quality and clean burning gasoline. Alkylate gasoline, which currently constitutes about 14% of the gasoline pool, is typically produced by alkylating isobutane with low-end olefins. Conventional alkylation processes use either sulfuric acid or hydrofluoric acid as catalyst, both of which have a number of drawbacks.
Disadvantages associated with the use of H2SO4 as an alkylation catalyst include the vast quantities of acid required to initially fill the reactor, and the large amounts of spent acid to be withdrawn on a daily basis for off-site regeneration, which involves incinerating the spent acid and preparing fresh acid. Disadvantages associated with the use of HF as an alkylation catalyst include the special handling requirements due to the highly corrosive nature of the acid, and the formation of aerosols, which presents an environmental and safety risk. These risks are evident from the additional safety measures associated with modern HF alkylation processes, such as water spray and catalyst additive for aerosol reduction.
Accordingly, catalyst systems that are safer and more environmentally friendly than HF and H2SO4 are required for refinery alkylation processes. However, thus far, no viable replacement catalyst systems have been commercialized, despite extensive research in both academic and industrial institutions.
Ionic liquids are liquids that are composed entirely of ions. Fused salt compositions are a class of ionic liquids that are liquid at low temperatures, with melting points often below room temperature. In general, such compositions have found applications as catalysts, solvents and electrolytes. The most common ionic liquids are those prepared from organic cations (ammonium, phosphonium, and sulphonium) and inorganic or organic anions. Anions of ionic liquids include BF4−, PF6−, haloaluminates such as Al2Cl7− and Al2Br7−, [(CF3SO2)2N)]−, alkyl sulfates (RSO3−), carboxylates (RCO2−). The most interesting ionic liquids for acid catalysis are those derived from organic halide salts and Lewis acids (such as AlCl3, TiCl4, SnCl4, FeCl3, etc). Chloroaluminate ionic liquids are perhaps the most commonly used ionic liquid catalyst systems for acid-catalyzed reactions.
Chloroaluminate ionic liquids can be prepared, for example, from an alkylpyridinium chloride or alkylimidazolium chloride and a metal halide. The use of the fused salts (1-alkylpyridinium chloride and aluminum trichloride) as electrolytes is discussed in U.S. Pat. No. 4,122,245. Other patents which discuss the use of fused salts as electrolytes include U.S. Pat. Nos. 4,463,071 and 4,463,072. Ionic liquids and their methods of preparation are also disclosed in U.S. Pat. Nos. 5,731,101; 6,797,853; 5,104,840 and in US Patent Application Publication Nos. 2004/0077914 and 2004/0133056.
During the past decade, the emergence of chloroaluminate ionic liquids has sparked some interest in AlCl3-catalyzed alkylation in ionic liquids as a possible alternative to conventional catalysts. For example, the alkylation of isobutane with butenes and ethylene in ionic liquids has been described in U.S. Pat. Nos. 5,750,455; 6,028,024; and 6,235,959 and in the Journal of Molecular Catalysis, 92 (1994), 155-165; “Ionic Liquids in Synthesis”, P. Wasserscheid and T. Welton (eds.), Wiley-VCH Verlag, 2003, pp 275). U.S. Pat. No. 7,531,707 to Harris et al. discloses a process for the alkylation of light isoparaffins with olefins using an ionic liquid catalyst and an alkyl halide promoter.
As a result of their use in catalytic reactions, ionic liquids become deactivated and may eventually need to be replaced. However, ionic liquid catalysts are expensive and replacement adds significantly to operating expenses by, in some cases, requiring shutdown of an industrial process. One of the heretofore unsolved problems impeding the commercialization of chloroaluminate ionic liquid catalysts has been the inability to effectively and efficiently regenerate and recycle them.
Recently, methods for ionic liquid catalyst regeneration were disclosed in US Patent Application Pub. No. 2007/0249485 (Elomari, et al.), in which spent ionic liquid catalyst, in combination with conjunct polymer, was reactivated by treatment with a regeneration metal. A consequence of this treatment is that excess metal halide may accumulate in the ionic liquid during catalyst regeneration. US Patent Application Pub. No. 2009/0163349 (Elomari, et al.), discloses the removal of excess metal halide from an ionic liquid catalyst by the addition of either an organic halide salt or a mixed salt, corresponding to the ionic liquid catalyst, having a metal halide/organic halide salt molar ratio less than two.
There is a need for methods for the regeneration of spent ionic liquid catalysts wherein the composition of the catalyst can be efficiently and economically maintained during and/or after catalyst regeneration. There is a further need for ionic liquid catalyzed alkylation processes wherein the catalytic activity and composition of the ionic liquid catalyst is stabilized in an economic and efficient manner.